1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to an in-plane switching (IPS) mode liquid crystal display device in which liquid crystal molecules are homogeneously aligned during black display, and an electrical field is applied to the molecules in a lateral direction, to thereby control light transmittance and blockage, and to improving the viewing-angle characteristics (particularly for black display and low gradation display) of such a device.
2. Description of the Related Art
As systems in which the direction of the electrical field applied to the liquid crystal is made parallel to the substrates (hereinafter, referred to as lateral electrical-field systems or IPS mode), systems employing a comb-tooth electrode provided on one substrate have been proposed in Japanese Patent Application Laid-open. Publication. No Sho 56-91277, Japanese Patent Application Laid-open No. Hei 9-80424, and Japanese Patent Application Laid-open No. 2001-056476. According to those systems, it has been known that the liquid crystal molecules primarily rotate in a plane horizontal to the substrate, and hence the difference in the degree of birefringence when viewed obliquely between periods when an electrical field is applied and periods when an electrical field is not applied is small, and the viewing angle is wide.
However, although changes in birefringence of the liquid crystal itself are small in the IPS mode, it has been known that, due to the characteristics of polarizers, light leakage occurs when viewed from an oblique direction aligned away from the absorption axis of the polarizers. A system which uses a retardation film in order to stop such light leakage of polarizers in the oblique direction is disclosed in Japanese Patent Application Laid-open No. 2001-350022. However, the publication basically aims to improve the viewing angle of the polarizers alone. The effect of the liquid crystal is considered for a vertical alignment (VA) mode, but a system for compensating for the effect of the liquid crystal layer in the IPS mode is not disclosed in the publication in any way.
Japanese Patent No. 3204182 discloses means for suppressing the occurrence of changes in white color depending on the viewing direction. However, there is no suggestion of the improvement of black display characteristics.
Meanwhile, disclosed in Japanese Patent No. 2982869 is a configuration in which a retardation film is positioned to the inside of one of the polarizers in order to improve the viewing-angle characteristics of black display. In this system, the effect of triacetylcellulose (TAC) forming supporting-substrate materials positioned on both sides of the polarizers is considered. However, the investigations of the inventors of the present invention revealed that phase compensation with one retardation film in only one polarizer does not provide sufficiently deep black display at oblique viewing angles, nor reduce coloring due to wavelength dispersion of the liquid crystal layer. Moreover, there is no disclosure of the difference in phase compensation which depends on whether the alignment axis (slow axis) of liquid crystal molecules during black display is parallel or perpendicular to the absorption axis of the polarizer on the input side. In the related art mentioned above, the viewing-angle characteristics are discussed only in the context of brightness characteristics, and changes in color are not addressed at all.
Further, Japanese Patent Application Laid-open No. 2005-208356 discloses a configuration in which a supporting-substrate material of one polarizer is substantially optically isotropic and a retardation film is provided in the other polarizer in order to improve a bright spot and coloring of black display at oblique angles. This system may reduce the effect of wavelength dispersion of the liquid crystal layer. However, the investigations of the inventors of the present invention revealed that this system does not reduce coloring due to wavelength dispersion of the retardation film.
Moreover, Japanese Patent Application Laid-open No. 2008-242041 discloses a configuration in which a color filter (CF) with different thickness-direction retardations Rth for colors of red (R), green (G), and blue (B) is provided in order to improve coloring of black display at oblique angles. However, there is no suggestion of the configuration of a retardation film necessary to improve the bright spot of black display at oblique angles. In addition, there is no disclosure of the difference in phase compensation which depends on whether the alignment axis (slow axis) of liquid crystal molecules during black display is parallel or perpendicular to the absorption axis of the polarizer on the input side.